The present invention relates to a high frequency heating apparatus, and more particularly to a high frequency heating apparatus equipped with an improved door arrangement in which leakage of high frequency energy from a high frequency shielding member of an observation window provided in the door is reduced.
Generally, in a high frequency heating apparatus, for example, in a microwave oven particularly having an electric heating capability in addition to the high frequency or dielectric heating capability and known as a microwavethermal oven, it is required to provide a glass plate member in front and at the outer side of the high frequency shielding member for reducing heat radiation from the heating chamber or heating cavity through the shielding member and maintaining the surface of the door at low temperatures. Especially, when soiling in the heating chamber of the microwave oven is to be removed by thermal decomposition to clean the oven, the temperature within the heating chamber rises close to approximately 500.degree. C., and thus, a plurality of glass plates are required to be installed for maintaining the outermost surface thereof at low temperatures.
For mounting the glass plates as described above in the door assembly, however, special arrangements become necessary particularly to prevent leakage of the microwave energy with respect to the shielding member. Although the microwave leakage through the shielding member which has substantially no movable parts does not increase, for example, due to variations with time, etc. as compared with the microwave leakage from gaps and the like between the heating chamber and the door, such microwave leakage, if large in amount, tends to cause the leakage value to exceed standard values because of a large local power density concentration by generation of standing waves resulting from relative interference thereof with respect to microwave leakage sources around the door.
In connection with the above, existing standards set by the government require that the leakage of microwave energy from microwave cooking appliances should be kept to minimum levels.
Accordingly, in the arrangements of microwave ovens on the whole, it is extremely important to suppress the microwave energy leakage from the shielded portion in the front of the door.
In view of the peculiar problems as described in the foregoing, there have heretofore been proposed microwave ovens, for example, in U.S. Pat. No. 4,028,519 patented on June 7, 1977 and in Japanese Laid Open Patent Application Tokkaisho No. 52-112842, each provided with a particular arrangement for preventing microwave energy leakage. The known arrangements as described above, however, still have a tendency to permit increased microwave energy leakage from the front portion of the door for the heating chamber, thus presenting problems to be further solved with respect to the microwave energy leakage.